1. Field of the Invention
The present invention relates to a fixing belt, sheet conveying belt, intermediate image transferring belt or similar belt member included in an image forming apparatus and a belt device, a sheet conveying device, a fixing device, an image transferring device, a duplex image transferring device and an image forming apparatus using the same.
2. Description of the Background Art
It is a common practice with an image forming apparatus to use a belt device in which an endless belt for, e.g., conveying a sheet or effecting intermediate transfer of a toner image formed on a photoconductive drum or similar image carrier is passed over a plurality of support rollers. One of the support rollers is implemented as a drive roller for driving the belt. A problem with such a belt device is that it is difficult to position the support rollers precisely horizontally or make the circumferential length of the belt member uniform in the widthwise direction thereof, resulting in the deviation of the belt member to one side in the axial direction of the support roller during the drive of the belt member.
In light of the above, Japanese Patent Laid-Open Publication No. 4-190280, for example, discloses an image forming apparatus including a belt member formed with circumferential ribs that adjoin opposite edges of the inside surface of the belt member. The ribs are configured to abut against the ends of a support roller to thereby prevent the belt member from being shifted and running off the support roller.
Japanese Patent Laid-Open Publication No. 10-268660, for example, teaches an image forming apparatus in which ribs, formed on the inside surface of a belt member, are received in annular grooves formed in a support roller in order to prevent the belt member from running off the support roller.
Today, the diameter of a support roller is deceasing to meet the increasing demand for the size reduction of an image forming apparatus. As a result, a belt member is often wrapped around a support roller with a large curvature 1/R, which is the reciprocal of a diameter. Further, a belt member sometimes must be pressed from the outside to be inversely bent toward the inside for the purpose of enhancing free layout around the belt member, as in an image forming apparatus proposed by Japanese Patent Laid-Open Publication No. 2003-177617.
However, if a belt member is wrapped around a support roller with a large curvature or inversely bent inward, then ribs formed on the belt member for the anti-deviation purpose are subject to a heavy load and easily cracked or otherwise deteriorated. Moreover, the ribs, thicker than the base of the belt member and difficult to bend, prevent the base from smoothly bending or inversely bending and thereby cause it to locally rise away from and therefore slip on the support roller.